The present invention relates to a clearance control arrangement, in particular, although not exclusively, a tip clearance control arrangement for controlling the clearance of rotor blades in casings in gas turbine engines.
A gas turbine engine 10 is shown in FIG. 1 and comprises an air intake 12 and a propulsive fan 14 that generates two airflows A and B. The gas turbine engine 10 comprises, in axial flow A, an intermediate pressure compressor 16, a high pressure compressor 18, a combustor 20, a high pressure turbine 22, an intermediate pressure turbine 24, a low pressure turbine 26 and an exhaust nozzle 28. A nacelle 30 surrounds the gas turbine engine 10 and defines, in axial flow B, a bypass duct 32.
Each of the compressors and turbines comprise alternating stages of rotating blades and stationary stators surrounded by a casing. Engine efficiency is improved as the clearance between the rotating blade tips and the casing is minimised so that the working fluid passes over the blade surfaces and does not leak over the tips. However, differential heating of the components occurs during operation of the gas turbine engine. For an aero gas turbine engine used to power an aircraft, the radial growth of the blades is quicker than the radial growth of the casing when the engine accelerates, for example during take off and climb manoeuvres. Thus the minimum clearance between the blade tips and the casing must be set for the worst case scenarios, e.g. take off and step climb. At other flight phases the blades radially shrink more than the casing so that the clearance is larger than optimal.
Selective case cooling has been used to decrease the clearance during aircraft cruise. However, the clearance cannot be wholly minimised because the casing must be able to radially grow quickly enough that the blade tips do not rub against the casing during step climb manoeuvres.
Another method of decreasing the clearance during cruise has been to provide radially moveable segments of casing. These are generally mechanically complex and heavy, requiring significant actuation components. Additionally a casing segment is a relatively large component to move which cannot respond rapidly enough to transient aircraft manoeuvres such as step climb. Thus the minimum clearance must be specified at all flight phases as that required for take off, climb and step climb meaning that the engine is more inefficient in cruise. This is particularly expensive as cruise generally comprises the greatest proportion of the flight, at least for passenger and freight aircraft.